The invention is particularly applicable to electrostatic coalescing devices for promoting the coalescence of water in an emulsion comprising oil and water. However, the invention is applicable to any type of coalescing application where it possible to promote the coalescence of an emulsion component in an emulsion comprising a mixture of at least two different fluid components by means of an electric field applied to the emulsion.
In the oil and gas industry where oil is extracted from one or more wells in an oil field, oil will be extracted together with water. The water has to be removed from the oil and this is mainly done by means of settling tanks in which the oil is permitted to settle under the action of gravity. However, stable oil-water emulsions may develop during the production of the oil. For example, the use of gas-liquid cyclones might contribute to a stable emulsion that will be difficult to separate only by means of settling. After having passed through, for example, a series of gravitational settling tanks, a certain amount of water normally remains in the oil in the form of droplets. In order to promote the separation of this remaining water content, which is difficult to separate from the oil only by means of further gravitational settling, different types of coalescing devices have been proposed taking advantage of the fact that water and oil have different permittivity.
It is well known to use electrostatic coalescing devices in order to achieve water droplet enlargement or coalescence of water in water-in-oil emulsions, whereupon the water can be separated more easily from the oil, e.g. by means of gravitational separation or the like. An electrostatic coalescing device can be employed to speed up the separation of any emulsion where the continuous phase is an electrical insulator, such as oil, and the dispersed phase has a different permittivity than said continuous phase. The dispersed phase may for instance be an electrical conductor, such as water. In an electrostatic coalescing device, an emulsion is subjected to an alternating current field or to a continuous or pulsed direct current field.
WO 03/049834 A1 discloses an electrostatic coalescing device comprising several planar sheet-shaped electrodes extending in parallel with each other so as to form flow passages for an emulsion between each pair of adjacent electrodes. Different electric potentials are applied to the electrodes so as to form an electric field between each pair of adjacent electrodes, which e.g. will promote the coalescence of water contained in a water-in-oil emulsion flowing through the flow passages between the electrodes. This system requires an external high voltage transformer and a high voltage connection between the transformer and the electrodes.
WO 2003/039706 discloses an electrostatic coalescing device comprising a number of tubular electrostatic coalescer elements. The tubular elements are extending in the flow direction and are arranged in a matrix substantially covering the entire cross sectional area of said vessel. An electrical field is applied to the fluids flowing through said coalescer elements. The electrodes may be insulated and energized trough a capacitive coupling from a central module. A complete transformer may be moulded into a central module. The high voltage secondary winding of the transformer may be insulated from the fluid, whereas the primary terminal winding is accessible from the outside.
WO 2007/135503 and WO 2007/135506 describes an electrostatic coalescing device comprising pairs of sheet-shaped electrodes arranged at a distance from each other side-by-side so as to form a flow passage between them. Each of the electrodes comprising a sheet-shaped conductive member of electrically conductive material, the mutual distance between the conductive members of the two electrodes of a pair varies along the electrodes as seen in a direction perpendicular to the intended flow direction of fluid passing through the flow passage between the electrodes. A power supply supplies mutually different electric potentials to the conductive members of the electrodes of said pair so as to form an electric field between the electrodes. An electrode pair is connected to an external transformer. High voltage connections are complicated, highly sensitive to fouling, bulky, and not flexible.